dr-ebert
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As Leibnitz said (and I quoted recently): let's calculate.that way the heat generated- READ: 10's to 100's of KILOWATTS PER SQUARE CENTIMETER power density- does not have to go through the bulk of the chip to get dissipated.
An LPC diode has an operating current of around 400mA the way it's being done here, the voltage drop over the diode is ~2.5V, so the power is just about 1W. Around 250mW is generated as laser light; that means 750mW are generated as heat.
From a picture in the internet, you can measure the dimensionso the LPC long die diode as approximately 0.3 x 0.015 x 0.01 cm (for the sake of familiarity, I'm not using SI units). The contact area with the brass case is therefor 0.0045 cm2 (sigh... still no sub/superscripts).
This translates to 0.75/0.0045 = 167W/cm2. Fully 3 orders of magnitude lower than heruursciences assertion.
The thickness of the die is ~0.1mm, like a thin piece of paper. How significant is a thermal gradient over such a small thickness? Again... you guessed it... math comes to the rescue.
Here you can find information on the thermal conductivity of Al(x)Ga(1-x)As (*sigh* again): depending of the value of x (i.e. the exact composition), it varies between 0.11..0.55 W/(cmK), which is pretty low - Silicium is 3 to 15 times better. All the waste heat has to be transferred through the contact area with the brass case (see above). The formula for the necessary temperature gradient is:
delta T [K] = P [W] x d [cm] / A [cm2] / k [W/cmK]
Plugging in the worst case for k (0.11 W/cmK) and the other numbers, we get (this also assumes that the heat gets generated on only the "far side" of the die so it has to be transported the max distance):
delta T = 15 K (or deg C)
This is for x=0.5; for pure GaAs (x=0) the thermal conductivity is 5 times better and delta T would be 3 K.
It seems unlikely that anyone would introduce major changes in the manufacturing process for such a meager difference - especially as the operating conditions (current) I cited are way above the rated spec of the diode; for them, the temp difference would be correspondingly less. So it's safe to conclude that we can exclude thermal considerations from the case positive/negative debate.